Conventionally, a laser welding method using a laser processing device is widely adopted.
When welding is performed using the laser processing device, spatters fly apart at a position to which a laser beam is applied, and may adhere to the laser processing device. For this reason, in the laser processing device, a protective glass is commonly arranged at the front end part of an optical system in order to protect the optical system from the spatters.
When the spatters adhere to the protective glass, the laser beam is partly blocked, which results in decrease of the power of the laser beam. For this reason, a technique for determining whether the spatters adhere to the protective glass is developed, and is disclosed in JP-A 2005-302827.
JP-A 2005-302827 discloses an apparatus including an independent applying device for applying light to a protective glass, which is reflected by the protective glass, and an independent receiving device for receiving the reflected light from the protective glass. The apparatus compares the intensity of the light received by the receiving device with a predetermined standard value, thereby determining whether the protective glass is dirty or not.
Intensive studies by the present inventors have shown that, when the spatters adhere to the protective glass, the focal length of the laser beam varies due to “thermal lens effect”. The “thermal lens effect” is a phenomenon in which the laser beam is absorbed by the spatters adhering to the protective glass, and the refractive index of the protective glass varies in the parts around the spatters.
The apparatus disclosed in JP-A 2005-302827 can determine whether the protective glass is dirty or not, but cannot determine how far the focal length of the laser beam varies due to the thermal lens effect.
For this reason, conventionally, if the focal length of the laser beam varies due to the thermal lens effect during laser welding, the focal length cannot be adjusted properly. Therefore, a cause of poor welding has been left unsolved.